A challenging issue confronting the air transport system is the demand for the reduction of the emissions of oxides of nitrogen. The formation of thermal NOx in a gas turbine engine depends on the stoichiometry, the residence time linearly, and on the reaction temperature exponentially. Zeldovich thermal NOx may be produced by oxidation of atmospheric nitrogen in post flame gases. As turbine blade resistance to high temperatures improves, nitrogen production in the post-combustion zone may become more important. While residence time is not as significant as temperature in formula predicting NOx production, it is a necessary factor and should be as accurate as possible. The characteristic combustor residence time can be defined as the ratio of the combustor volume to the bulk (volumetric) flow rate. This value is estimated from geometry and operational data. Detailed geometrical and operational data from gas turbine engine manufacturers, however, is frequently unavailable. Furthermore, post-combustion residence time measurements are not available to verify analytical estimates. Consequently, in order to improve the technology to satisfy future emission prediction goals, a different concept for determining the characteristic post-combustor residence time is required.